BY JOHN THOMSON, M.B. 45 



Abney employs what he calls a colour-patch-apparatus, a 

 contrivance by which rays from sun- or arc-light are first 

 parallelised and then partially refracted and partially reflected ; 

 the former, the refracted, pass through two prisms and are 

 focussed on a screen to form a spectrum, or by using a slit, any 

 ray of a spectrum, or by employing a collective lens, a white 

 image ; the latter, the reflected, fall on a mirror and by means 

 of a lens can be thrown on the same screen and alongside the 

 colour patch.* 



In cases of acquired colour-blindness, a condition I have 

 already said I have no experience of, with the affected retinal 

 area or areas small, the wool test would fail, for the skeins being 

 large their images would be received on parts of the retina not 

 diseased and colours would be correctly matched. 



To provide for this Sir Wm. Abney says: "I have had a 

 set of brick-clay pellets some f^j^ inch in diameter, painted with 

 water colours mixed with soluble glass solution of the same 

 colours as the wools. These are placed in a shallow tray and 

 presented to patients affected with this central blindness to pick 

 ■out all the pellets which match reds and greens. They will tell 

 you they see neither the one nor the other, though they will pick 

 out the blue pellets unerringly. A red pellet they will match 

 with a red, green, grey or a brown one, and a green one with 

 the same. If, however, you instruct them to direct their eyes 

 a few degrees away from the tray, they will tell you they see all 

 the colours, and as they endeavour to pick thorn out, they, with 

 a natural instinct, direct their eyes again to the collection when 

 once more the colours vanish. It is almost piteous sometimes 

 to see the distress which this simple test occasions." 



Watching a colour-blind individual stumbling over his mis- 

 taken skeins is a rather painful sight, but there are occasionally 

 some amusing incidents. 



Examinee 4 was a fine strapping fellow, and I was anxious 

 to get him for the Railways, and I was also anxious to experi- 

 ment with him to discover if possible any method which might 

 be adopted to correct his defective colour sense. Time and again 



* Note. — I referred to a leading, perhaps the leading, firm of opticians 

 in London, and received answer : — " 7/9/03. The only coloured tests that 

 seem to be in demand are the ordinary sets of wools. Nearly all the tests 

 you mention in your letter have not been manufactured for many years ; 

 •we could let you have them if you wish, but it is doubtful whether you 

 would consider that they would be worth the carriage to send." 



